1. Field
The present invention relates to a patterning device for a lithographic apparatus.
2. Background
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate.
In an EUV lithographic apparatus extreme ultra-violet (EUV) radiation is used for imaging. This radiation has a wavelength in the range of 5-20 nm. The use of refractive optical elements in EUV lithography machines is not feasible due to absorption in bulk material of EUV radiation. Consequently, for EUV lithography reflective optical systems are used for imaging, and reflective reticles are used for patterning. A reflective reticle includes a pattern of reflective and absorbing (or at least less reflective) areas on one surface (the “front surface”) of the reticle. The pattern is generated using an electron beam (“e-beam”) writing tool. To avoid electron-beam writing-errors due to electrostatic fields near the front surface of the EUV reticle during patterning the EUV reticle, the reticle front surface is electrically grounded during e-beam writing. To that purpose a conductive coating is provided to an area at the front surface. An electric conductor, connected to earth, is brought in contact with the frontside conductive coating, to establish the required electrical grounding of the reticle.
Not only during patterning the reflective reticle, but also when in use in an EUV lithography apparatus the reflective reticle is electrically grounded. Impact with EUV photons may cause electrons to be freed from the reticle (for example, from the reticle substrate or its surface and/or from an absorber layer or a reflective multi-layer) and as a result a subsequent electrostatic charging of the substrate takes place. Electrical grounding is applied to avoid negative effects due to an electric field caused by electrostatic charging of the reticle. In an EUV machine a reticle support is capable of clamping the reflective reticle at the backside of the reticle (i.e. the side facing the side provided with the pattern). The reticle may be held in place by means of electrostatic clamping of the reticle to a chuck which is part of the reticle support. During electrostatic clamping a conductive coating at the backside of the reticle is held at electrical ground, and electrostatic clamping forces are generated by applying a voltage between this conductive coating and a conductive layer inside the chuck. For example, with the coating of the reticle at electric ground level, the conductive layer in the chuck may be held at 1000 V.
To obtain electrical grounding of the reticle, an electric conductor connected to earth may be brought into contact with the conductive coating at the front surface of the reticle and/or a conductive coating used for electrostatic clamping of the reticle. The conductor may for example be embodied as a spring-loaded metal pin. A finite force is needed to bring and keep the conductor or pin in contact with the conductive coating such as to obtain a sufficiently low contact resistance, and consequently the conductor may slightly damage the coating and may generate particles, which should be avoided in both the electron beam writing tool and in the EUV lithographic apparatus.